1. Field of the Invention
The present invention relates to a scanning image forming lens and an optical scanning apparatus.
2. Description of the Related Art
An optical scanning apparatus for optically scanning a surface to be scanned at a constant velocity by deflecting a luminous flux emitted from a light source via an optical deflector so as to transmit the luminous flux through a scanning image forming lens to be condensed into an optical beam spot on the surface to the scanned is well known in an image forming apparatus such as a laser printer, a digital copier, a facsimile machine and other such devices. The luminous flux is deflected in a direction corresponding to a main scanning direction for the surface to be scanned at equiangular velocity by rotation of the optical deflector such that the optical beam spot formed by the scanning image forming lens scans the surface to be scanned at a constant velocity in the main scanning direction. Typically, the surface to be scanned includes a photoconductor and the optical beam spot forms picture elements which constitute an image to be formed on the surface to be scanned.
The above-mentioned main scanning direction refers to the direction corresponding to the main scanning direction for a surface to be scanned along a light path from a light source to the surface to be scanned. A direction corresponding to a sub scanning direction refers to the direction corresponding to a sub scanning direction for the surface to be scanned along the light path. The sub scanning direction is substantially perpendicular to the direction corresponding to the main scanning direction.
In such an optical scanning apparatus, when the optical beam spot scans the surface to be scanned in the main scanning direction, if the diameter of the optical beam spot changes depending upon the position of the optical beam spot in the main scanning direction on the surface to be scanned, the size of each picture element, which is formed by the optical beam spot on the surface to be scanned so as to form an image on the surface to be scanned, changes depending upon the position where each picture element is written on the surface to be scanned in the main scanning direction. Consequently, the resolution of the formed image changes along the main scanning direction, resulting in deteriorating of image quality. A position on the surface to be scanned in the main scanning direction is sometimes referred to herein xe2x80x9cimage heightxe2x80x9d.
A change in the diameter of the optical beam spot in the main scanning direction on the surface to be scanned according to the position of the optical beam spot in the main scanning direction can be corrected to a certain degree, for example, by adjusting the time for writing each picture element on the surface to be scanned with the optical beam spot. However, a change in the diameter of the optical beam spot in the sub scanning direction according to the position of the optical beam spot in the main scanning direction cannot be corrected by adjusting the writing time for the optical beam spot. The change in the diameter of the optical beam spot in the sub scanning direction according to the position of the optical beam spot in the main scanning direction is typically corrected through adjustment of the optical performance of an optical lens system used for transmitting the light flux, which is deflected by the optical deflector in the direction corresponding to the main scanning direction, such that the luminous flux is condensed into an optical beam spot on the surface to be scanned. The optical lens system for transmitting the deflected light flux so as to form an optical beam spot and to scan the surface to be scanned with the optical beam spot is herein called a scanning image forming lens.
It is known that a change of an optical beam spot diameter in the sub scanning direction on a surface to be scanned according to the image height can be suppressed by correcting the curvature of field of the scanning image forming lens in the sub scanning direction. Typically, correction of the curvature of field in the sub scanning direction is made only relative to a paraxial luminous flux of the scanning image forming lens system.
However, correction of the curvature of field of the scanning image forming lens must be performed while keeping the other optical characteristics of the scanning image forming lens at a satisfactory level. For example, the constant velocity characteristics must be kept at a satisfactory level for enabling the optical beam spot to be moved at a constant velocity along the surface to be scanned. Typically, when the curvature of field is corrected, the other optical characteristics deteriorate.
In addition, when, for example, a photoconductor is used in an optical scanning apparatus as the surface to be scanned for forming an image thereupon, an assembly tolerance of the photoconductor in relation to the scanning image forming lens must be carefully considered in the design of the apparatus for accomplishing a desired level of the curvature of field in the scanning image forming lens. More specifically, even when the curvature of field of the scanning image forming lens is accurately corrected in the design of the scanning image forming lens, the curvature of field of the scanning image forming lens as designed may not necessarily be realized if the actual position of the surface to be scanned in relation to the scanning image forming lens is deviated from the designed position due to, for example, an assembling error relating to the position of the surface to be scanned relative to the scanning image forming lens.
In order to overcome the problems described above, preferred embodiments of the present invention provide a scanning image forming lens and an optical scanning apparatus that accurately correct any changes in diameter of an optical beam spot in a sub scanning direction according to the image height.
The preferred embodiments of the present invention also provide a scanning image forming lens and an optical scanning apparatus that limit any changes of the diameter of an optical beam spot according to the image height within a predetermined range in a sub scanning direction when a surface to be scanned is positioned within a predetermined range of assembling tolerance relative to the scanning image forming lens.
A scanning image forming lens system according to a specific preferred embodiment of the present invention is preferably used in an optical scanning apparatus for optically scanning a surface to be scanned by deflecting a luminous flux emitted from a light source in a direction corresponding to a main scanning direction via an optical deflector at equiangular velocity. The scanning image forming lens system transmits the luminous flux deflected by the optical deflector so as to condense the luminous flux into an optical beam spot on the surface to be scanned and to scan the surface to be scanned with the optical beam spot.
The scanning image forming lens system according to preferred embodiments of the present invention includes one or more image forming optical elements. The one or more image forming elements preferably includes at least one lens having at least one lens surface that preferably has a non-arc shape in a sub scanning cross section. The non-arc shape changes according to a position in a direction that is substantially perpendicular to the sub scanning cross section (i.e., a position in a direction corresponding to a main scanning direction, which is referred to as an image height) such that a positional deviation of the optical beam waist of the deflected luminous flux from the surface to the scanned at each position in the direction that is substantially perpendicular to the sub scanning cross section, which is caused by a paraxial curvature of field of the scanning image forming lens system in a sub scanning direction, is corrected.
The above-mentioned sub scanning cross section herein refers to a flat cross section which is substantially perpendicular to the direction corresponding to the main scanning direction. An optical beam waist herein refers to a portion of a luminous flux having a minimum diameter. A paraxial curvature of field in a sub scanning direction is caused by a paraxial luminous flux with respect to a scanning image forming lens system, and is typically called a curvature of field in the sub scanning direction. Further, a lens surface having a non-arc shape in a sub scanning cross section and in which the non-arc shape changes according to a position in a direction perpendicular to a sub scanning cross section such that a positional deviation of the optical beam waist of a deflected luminous flux from a surface to be scanned at each position in the direction that is substantially perpendicular to the sub scanning cross section is corrected is herein called a xe2x80x9cbeam waist position correcting lens surfacexe2x80x9d for the convenience of explanation.
In the above-described configuration, the scanning image forming lens system may include two or more lenses. Further, the scanning image forming lens system may include a reflective mirror having a focusing function in addition to the lens. For example, the scanning image forming lens system may include a combination of one or more such reflective mirrors and one or more lenses.
Further, the beam waist position correcting lens surface may be formed at more than two lens surfaces in the scanning image forming lens system. In such a case, the beam waist position correcting lens surface may be formed, for example, at both surfaces of one lens that is included in the scanning image forming lens system, or at one surface of each of the two lens that are included in the scanning image forming lens system.
According to the above-described preferred embodiments, a change in a diameter of an optical beam spot in a sub scanning direction on a surface to be scanned, according to a position of the optical beam spot in a main scanning direction, can be maintained, for example, within a range of about plus and minus 5%, regardless of the existence of a paraxial curvature of field in the sub scanning direction in the scanning image forming lens system. A diameter of an optical beam spot formed by a luminous flux herein refers to the diameter of a part of the optical beam spot where the light intensity is exe2x88x922 wherein e=2.71828.
In another preferred embodiment, the non-arc shape of the at least one lens surface of the at least one lens included in the scanning image forming lens system may be configured so as to change according to the position in the direction that is substantially perpendicular to the sub scanning cross section such that a change in a diameter of the optical beam spot in a direction corresponding to the sub scanning direction according to the position in the direction that is substantially perpendicular to the sub scanning cross section is maintained within a predetermined range when an assembling tolerance of the surface to be scanned relative to an originally designed position for the surface to be scanned is within a predetermined tolerance range.
Further, the non-arc shape of the at least one lens surface may be configured such that a paraxial curvature center line plotting a paraxial curvature center of the non-arc shape of the at least one lens surface of the lens has a curved line in a main scanning cross section. A main scanning cross section herein refers to a flat cross section including an optical axis of the lens surface having the above-described non-arc shape and which is substantially parallel to the main scanning direction.
Furthermore, the non-arc shape of the at least one lens surface of the lens that is included in the scanning image forming lens system may be formed asymmetrically relative to the optical axis of the lens having the non-arc shape. With such an asymmetrical configuration of the non-arc shape, when a polygonal mirror is used as the optical deflector, the influence of a so-called sag condition of the polygonal mirror on a change in the diameter of an optical beam spot in the sub scanning direction is greatly reduced.
The at least one lens that is included in the scanning image forming lens system and that has the at least one lens surface having the non-arc shape in the sub scanning cross section may preferably be formed of a plastic material, because the non-arc shape can be relatively easily formed with plastic using a molding process.
The above scanning image forming lens system may preferably have a function to enable the luminous flux which is deflected by the optical deflector at equiangular velocity to scan the surface to the scanned at a constant velocity, and a function to establish a conjugate relationship in a geometric-optic manner between a position near a deflecting point of the optical deflector and a position near the surface to be scanned in the direction corresponding to the sub scanning direction.
In another preferred embodiment in which the scanning image forming lens system includes three lenses, one of the three lenses that is located nearest the surface to be scanned along the light path may include the at least one lens surface that has the non-arc shape in the sub scanning cross section, with the non-arc shape changing according to the position in the direction substantially perpendicular to the sub scanning cross section. The at least one lens surface that has the non-arc shape in the sub scanning cross section may be located at a side of the surface to be scanned or at a side of the optical deflector.
According to one specific preferred embodiment of the present invention, an apparatus includes a light source for outputting light, a first lens system arranged to receive the light output from the light source and to transmit a light flux therefrom, an optical deflector arranged to receive the light flux from the first lens system and to deflect the light flux from a surface therefrom, and a second lens system arranged to receive the light flux deflected from the optical deflector and to condense the deflected luminous flux into an optical beam spot on a surface to be scanned so as to form images having image heights, the luminous flux condensed by the second lens system into the optical beam spot including an optical beam waist, the second lens system including a scanning and image forming element including at least one surface including a plurality of portions each having a non-arc shape in a sub-scanning direction such that at least two of the non-arc shapes are different from each other.
According to another specific preferred embodiment of the present invention, an apparatus includes a light source for outputting light, a first lens system arranged to receive the light output from the light source and to transmit a light flux therefrom, an optical deflector arranged to receive the light flux from the first lens system and to deflect the light flux from a surface therefrom, and a second lens system arranged to receive the light flux deflected from the optical deflector and to condense the deflected luminous flux into an optical beam spot on a surface to be scanned so as to form images having image heights, the luminous flux condensed by the second lens system into the optical beam spot including an optical beam waist, the second lens system including a scanning and image forming element including at least one surface having a plurality of portions each having a non-arc shape in a sub-scanning direction such that a beam waist of the entire luminous flux is located at a surface to be scanned for all image heights.
In another preferred embodiment of the present invention, an apparatus includes a light source for outputting light, a first lens system arranged to receive the light output from the light source and to transmit a light flux therefrom, an optical deflector arranged to receive the light flux from the first lens system and to deflect the light flux from a surface therefrom, and a second lens system arranged to receive the light flux deflected from the optical deflector and to condense the deflected luminous flux into an optical beam spot on a surface to be scanned so as to form images having image heights, the luminous flux condensed by the second lens system into the optical beam spot including an optical beam waist, the second lens system including a scanning and image forming element including at least one surface having a plurality of portions each of which contains a non-arc shape in a sub-scanning direction such that a minimum beam spot diameter in the sub-scanning direction is located at a surface to be scanned and comprises a least circle of confusion of the entire luminous flux.
In a further preferred embodiment of the present invention, an apparatus includes a light source for outputting light, a first lens system arranged to receive the light output from the light source and to transmit a light flux therefrom, an optical deflector arranged to receive the light flux from the first lens system and to deflect the light flux from a surface therefrom, and a second lens system arranged to receive the light flux deflected from the optical deflector and to condense the deflected luminous flux into an optical beam spot on a surface to be scanned so as to form images having image heights, the luminous flux condensed by the second lens system into the optical beam spot including an optical beam waist, the second lens system including a scanning and image forming element including at least one surface having a plurality of portions each of which contains a non-arc shape in a sub-scanning direction such that defocusing lines for a plurality of image heights are substantially coincident with each other.
Another preferred embodiment of the present invention includes a light source for outputting light, a first lens system arranged to receive the light output from the light source and to transmit a light flux therefrom, an optical deflector arranged to receive the light flux from the first lens system and to deflect the light flux from a surface therefrom, and a second lens system arranged to receive the light flux deflected from the optical deflector and to condense the deflected luminous flux into an optical beam spot on a surface to be scanned so as to form images having image heights, the luminous flux condensed by the second lens system into the optical beam spot including an optical beam waist, the second lens system including a scanning and image forming element including at least one surface having a plurality of portions each of which contains a non-arc shape in a sub-scanning direction such that all beam spot diameters are within a range for all image heights.
These and other elements, features, and advantages of the preferred embodiments of the present invention will be apparent from the following detailed description of the preferred embodiments of the present invention, as illustrated in the accompanying drawings.